A(1) Field of the invention
The invention relates to a digital-to-analog converter (D/A converter for short), for converting a digital signal consisting of a sequence of code words each comprising a plurality of bits into an analog signal.
A(2). Description of the prior art
D/A converters are used in mutually very different technical fields. Generally speaking they convert a digital signal, formed by a sequence of code words each comprising a certain number of bits and occurring at a certain sampling frequency f.sub.s into a time-continuous signal. To enable this conversion each code word is converted into a signal sample whose amplitude is proportional to said code words and constant during a sampling period T which is equal to 1/f.sub.s. This furnishes a staircase signal which is usually also applied to a low-pass filter at the output of which the desired analog signal occurs. Embodiments of D/A converters are shown in reference 1. Many D/A converters are implemented from a plurality of resistors through which accurately defined currents flow. The number of currents required for decoding a code word is at least equal to the number of bits of which this code word consists. If these code words have been obtained by encoding a speech signal they usually do not comprise more than 12 bits. If, however, they are obtained by encoding a music signal and hi-fi quality is required after decoding then they comprise, for example, 16 bits. The more bits a code word comprises the more accurate the resistors and the currents must be and the higher the complexity and the price of such a D/A converter. This price inhibits the use of these D/A converters in, for example, audio equipment such as magnetophones, for the consumer's market.
Hereinafter a code word consisting of, for example, d bits will be denoted by "d-bit code word". In a similar manner a D/A converter arranged for decoding a d-bit code word will be denoted by "d-bit D/A converter".